1. Field of the Invention
This invention relates to the catalytic reforming of naphtha. More particularly, it relates to the catalytic reforming of light and heavy fractions of naphtha. This invention especially relates to the fractionation of naphtha and the catalytic reforming of the several fractions in a fashion which provides an increased BTX yield.
2. Description of the Prior Art
The art of reforming naphtha hydrocarbons boiling in the gasoline boiling range has been practiced in one form or another for many years. Over these years the reforming process has developed to include regenerative and semi-regenerative operations in combination with operations wherein the total naphtha charge is passed sequentially through a plurarity of separate catalyst beds or separate fractions thereof are passed through one or more beds of reforming catalyst under conditions of operating temperature, pressure and space velocity considered most suitable for achieving desired reforming reactions. More recently reforming processes have been developed where the catalyst is regenerated continously.
When hydrocarbons boiling in the gasoline boiling range come in contact with the dual functional catalysts employed in reforming, a number of reactions takes place which include dehydrogenation of cycloparaffins to form aromatics, dehydrocyclization of paraffins to form aromatics, isomerization reactions and hydrocracking reactions. A typical dual functional catalyst contains a metallic hydrogenation-dehydrogenation catalyst, typically 0.1 to 1.0 weight % Pt which is dispersed on an oxide, acidic catalyst such as alumina. These bring about dehydrogenation and isomerization of saturated parraffins. When the reforming conditions are quite severe, coke formation in the catalyst occurs with consequent deactivation of the catalyst. Thus, it is quite apparent that the composition of the naphtha charge will necessarily influence the severity of the reforming conditions employed to produce a desired product. However, the reforming operations, as we know them today, have certain built in limits because of reaction kinetics, catalysts available and equipment to perform the reforming operation. With the advent of unleaded gasoline requirements a renewed interest has been generated to further adapt the reforming operation of the production of high octane unleaded reformate gasoline product.
The reforming art has suggested splitting a wide-boiling range petroleum fraction, often a full range or wide cut naphtha (100-430.degree. F.), into a lighter cut and a heavier cut and separately reforming the two cuts using optimium operating conditions to provide a particularly useful reformate. Such a split feed reforming process is disclosed in U.S. Pat. Nos. 3,432,425 of Bodkin et al., 3,753,891 of Graven et al. and 4,002,555 of Farnham et al. In all three of these processes the light fractions and heavy fractions are separately reformed in parallel fashion although each fraction may be processed in a series of catalytic reactors. Bodkin et al. reforms the light fraction under more severe conditions then the heavy fraction while Graven et al. employs the opposite concept. Farnham uses a cascade hydrogen system so that the heavy fraction is reformed at a higher pressure than the light fraction.
Although the naphtha feed is split into two fractions in U.S. Pat. No. 3,647,679 of Kirk, Jr. et al., parallel reforming is not employed. Rather, the light naphtha is catalytically reformed serially in a number of reactors with the heavy fraction added to the feed entering the last reactor in the series. The light fraction has a boiling range below about 390.degree. F. while heavy fraction boils in the range of about 390.degree. to about 415.degree. F. The process is said to upgrade heavy naphtha without excessive coke formation and to provide an increased amount of reformate of increased octane.
It is sometimes required to provide a reformate having particularly desirable properties for a special use or a reformate of a specifically useful composition, such as a reformate enriched in a benzene-toluene-xylene (BTX) fraction. This BTX fraction serves as the feedstock for a host of petrochemicals which are eventually transformed into fabrics, resins, molded products, films and a variety of other household and commercial products. U.S. Pat. No. 4,222,854 of Vorhis, Jr. et al. discloses a reforming process for the production of motor gasoline and BTX-enriched reformate by fractionating a naphtha feedstock into a mid-boiling BTX-precursor fraction, a high-boiling fraction and a low-boiling fraction. The BTX-precursor fraction is catalytically reformed in a first reforming zone to provide the BTX-enriched reformate while the high-boiling and low-boiling fractions are combined and catalytically reformed in a second reforming zone to provide motor gasoline.
It is an object of this invention to provide a reformate enriched in BTX content.
It is another object of this invention to provide an improvement in a conventional reforming process wherein the reformate will have a higher BTX content than is obtained in conventional naphtha reforming.
It is a further object of this invention to provide a reformate having an enhanced BTX composition utilizing conventional reforming catalysts, equipment and operating conditions.
The achievement of these and other objects will be apparent from the following description of the subject invention.